Rotors of gas turbines, in particular aircraft gas turbines, comprising a rotor disc and blades arranged in circumferential direction around the disc are widely known from practical experience. Typical blades are inserted with a dovetail shaped blade root into also dovetail shaped blade mounting areas on the disc. The dovetail of a turbine blade typically includes corresponding pairs of upper and lower dovetail lobes or tangs in a fir tree configuration. Correspondingly, the perimeter of the rotor disk includes a row of axial dovetail slots defined between corresponding disk posts having complementary upper and lower supporting lobes or tangs.
In order to make the blade mounting areas durable and failsafe it is known to strain-harden the surface of the blade mounting areas by shot peening. With this method, a for example ball-shaped shot peening media is blasted or peened, respectively, with high velocity against the surface of the blade mounting area. Said method allows the improving of the mechanical features of the component and the inducing of inherent compressive stress into the surface of the component such that the danger of crack formation and propagation in the component is reduced, and the fatigue strength and the life time of the blade mounting areas are increased.
Due to the contour of the blade mounting area profile like a dovetail or a fir-tree profile, there is a very limited accessibility for a shot peening device. Currently, shot peening of blade mounting areas is performed by using nozzles which are fitted on specific retaining fixtures and peen the blade mounting areas from the outside with a shower-like media stream. During the peening, the peening stream of the nozzles is directed substantially perpendicular to a slot bottom of the blade mounting area. Due to the unique contour shape of the blade mounting area, several areas are hit very well by the peening stream, however some very poorly. An area of difficult accessibility are the pressure flanks of the blade mounting area which are hit by the shot peening media in particular via rebounds from the slot bottom. Therefore, peening with standard outside peening nozzles from outside the component results in a non-uniform peening treatment since the slot bottom and the pressure flange are unequally exposed to the peening stream. In particular for dovetail profiles the unequal alignment of the pressure flange and the slot bottom against the peening stream is a determining factor.
For providing sufficient strengthening of the pressure flanks with shot peening, it is known to peen the slot bottom very intensively, so that the pressure flanks are peened by the rebounds from the slot bottom with a desired intensity. However, this method may result in a damaging of the slot bottom. Further, no uniform shot peening treatment of the whole blade mounting area in terms of peening intensity, peening coverage, originated surface roughness and residual stresses in the component surface is possible due to the unfavorable hit angle of the peening stream onto the blade mounting area surface.
From DE 10 2008 010 847 A1 it is known to shot peen blades of blade integrated discs (blisks) with a dual-nozzle unit from a suction side and a pressure side of the blade in one step. For making that possible each peening nozzle has a planar deflection area arranged with an angle to a longitudinal axis of each peening nozzle which redirects peening shot media from moving mainly along the longitudinal axis to a direction essentially vertically to the longitudinal axis. Therefore, the peening shot media can be shot from both sides simultaneous against the both sides of the surface of the blade.
Ultra-Sonic-Peening is another possible method to strengthen the surface of a blade mounting area. With this method, the treated component surface and a chamber create a hermetically sealed area where an Ultra-Sonic-Peening source imparts a small volume media stream on the part surface randomly, thereby strengthening it. It is a disadvantage of this method that material damages may occur due to the rather big size of the peening elements which may cause deformations in areas with thin material. Further, applying this method is quite costly as for each type of contour a separate chamber is required.